Dryer performance indicator

ABSTRACT

A cross-machine dryer performance indicator wherein the deviation in the pre-dryer moisture content of a material from the average cross-machine moisture is compared at multiple locations across the width of the dryer with a deviation in the post-dryer moixture content of the material from the average cross-machine moisture of the material leaving the dryer. The sense of the pre-dryer moisture deviation is compared with the sense of the post-dryer deviation to indicate the drying performance of the dryer. The width of the material is scanned to measure the average moisture in the material both before and after drying to provide an indication of the performance profile of the dryer. A readout of dryer performance profile indicates whether the dryer&#39;&#39;s performance is good, poor or indeterminate across its width.

United States Patent 1 1 Hickman et al.

1 51 May 8, 1973 [54] DRYER PERFORMANCE INDICATOR [73] Assignee: Industrial Nucleonics Corporation,

Columbus, Ohio 22 Filed: Oct. 5, 1967 21 Appl. No.: 673,129

OTHER PUBLICATIONS Curtiss Wright Measurement and Control News Feb., 1962.

' CONTROLLER 36d T Primary ExaminerRichard C. Queisser Assistant Examiner-Ellis J. Koch Attorney-William T. Fryer, III, C. Henry Peterson and James J. OReilly [57] ABSTRACT A cross-machine dryer performance indicator wherein the deviation in the pre-dryer moisture content of a material from the average cross-machine moisture is compared at multiple locations across the width of the dryer with a deviation in the post-dryer moixture content of the material from the average crossmachine moisture of the material leaving the dryer. The sense of the pre-dryer moisture deviation is compared with the sense of the post-dryer deviation to indicate the drying performance of the dryer. The width of the material is scanned to measure the average moisture in the material both before and after drying to provide an indication of the performance profile of the dryer. A readout of dryer performance profile indicates whether the dryers performance is good, poor or indeterminate across its width.

22 Claims, 8 Drawing Figures SCAINNING 7 E O s 22 2 5 E DRYER J1 3 4 I $1] 3 E IOU i I W PRE DRYER POST DRYER w a W '6 TARGET TARGET 1 I V HEAT B 261 1 28 SUPPLY 5 SA MO1STURE A u N IT MOISTURE DATA CONTENT w PROCESSOR COMPUTER UNlT 24 DRYING PERFORMANCE 30V INDICATOR May 8, 1973 Patented 3,731,520

3 Sheets-Sheet 1 scANNING CONTROLLER ssj [O 32 2 a I n 7 I 6 I 22 20 "I. 5 E I 4 i I8[.%,] 3 IO 9 j w PRE DRYER POST DRYER Iz w a W 6 TARGET TARGET L J A HEAT I B 261 1 28 A I SUPPLY 5 SA MOISTURE j UNIT MOISTURE DATA 7' U. CONTENT PRocEssoR -coMPUTER UNIT B I MOISTURE I 24 DRYI N G coNTENT PERFORMANCE 30w INDICATOR 42 I 2 3 4 5 s 1 e PRE- DRYER L I I l I I 3 IIIIIII TARGET, T

PosT-DRYER TARGET. T

GOOD DRYING MOISTURE I PROFILE MACH I N E DIRECTION KPFIEDRYER MOISTURE PRE-DRYER ,AvERAGE POST-DRYER I I [MOISTURE I POST-DRYER AVERAGE INDETER-- MINATE R/cH/Im w. H/ckMA/v A/I/TO/V/ A. ROCHELEAU 0A W0 A. s xrz lNI/E/VTORS DRYER PERFORMANCE INDICATOR BACKGROUND OF THE INVENTION This invention relates generally to material manufacturing processes and more particularly to a method and means for indicating the drying performance of a dryer for removing moisture from a material.

It has been difficult to estimate the drying performance of a dryer. Some prior systems have computed a figure for dryer efficiency from the amount of water removed relative to the amount of steam supplied to the dryer. Such system is described in an article by E. S. Savas, Computer Control in the Paper Mill, TAPPI Magazine, May 1964 (Pages 129A and 130A). U. S. Pat. No. 2,767,484 issued on Oct. 23, 1956 to P. C. Gilson describes a method for regulating a dryer in accordance with the measured shrinkage occurring in the product being dried. water removal control system for a paper making machine is described in U. S. Pat No. 2,922,475 issued on Jan. 26, 1960 to F. M. Alexander and assigned to the same assignee as the present invention. U. S. Pat. No. 3,260,642, issued July 12, 1966 to W. H. Canter, .lr., assigned to the same assignee as the present invention, describes a system for controlling the steam supply to a dryer in accordance with the moisture content measured before and after the final dryer. None of these prior art methods provides a reliable indication of how well a dryer is performing in removing moisture from a product, particularly in the cross-machine direction.

SUMMARY OF THE INVENTION The present invention provides a method and apparatus for measuring the performance of a dryer by comparing the moisture in the partially-dried product leaving the dryer with the moisture in the wet product entering the dryer. The deviations of the water content from. reference values, such as preselected target values, are computed. The relative sense of these deviations is compared in accordance with the following logic. If the -wet product enters the dryer with excess moisture and leaves with a moisture content below the post-dryer target, a good indication is registered. Conversely, if the product enters the dryer relatively dry and leaves wetter than the post-dryer target, a poor indication is registered. Any other set of conditions results in an indeterminate indication. A reliable index of measuring dryer performance is established.

In a specific embodiment, the performance ofa sheet dryer is provided by scanning gauges movable across the sheet before and after the dryer. The output signals of the scanning gauges are averaged to determine the average moisture existing across the width of the sheet. In a specific embodiment, average sheet moisture values are derived and used as reference values in substitution for the preselected targets. Logic circuitry determines the sense of pre-dryer moisture deviation from its average value relative to the sense of the postdryer-moisture deviation from its average value and effects one of the above three indications of dryer performance from a display device. Apparatus is also provided to indicate not only whether performance is good or bad but also to what degree the performance is acceptable (or unacceptable).

In addition, the dryer performance profile, i.e., the operating performance of the dryer at all points across its width, may be checked and read out on a suitable indicator such as an array of colored lights or an x-y recorder. Operating personnel may conveniently moni' tor the operation of the dryer by observing the performance display provided. Manual or automatic adjustments may be made to the dryer or other equipments to correct when any deterioration from a desired performance is indicated.

Accordingly, it is a primary object of the present invention to provide a reliable indication of dryer performance.

It is another object of this invention to provide an in dication of a performance profile for a dryer.

It is still another object of the invention to provide a data processor which logically reduces sheet moisture data to measure dryer performance.

BRIEF DESCRIPTION OF THE DRAWING FIG. I is a block diagram of an industrial sheet dryer showing moisture gauges and an associated data processing system for carrying out the method of the present invention;

FIG. 2 is a graph of drying curves illustrating contrasting dryer performance characteristics;

FIG. 3 is a plot of sheet moisture content measured in a cross-sheet direction, before and after the dryer, shown in FIG. 1,;

FIG. 4 is a block diagram, partly schematic, of one type of data processor useful in the system shown in FIG. 1;

FIG. 5 is a block diagram of an alternative data processor;

FIG. 6 is a block diagram illustrating one method for indicating dryer performance profile; and

FIG. 7 is a block diagram of an alternative dryer performance profile indicator.

FIG. 8 shows a slightly modified portion of FIG. I combined with a portion of FIG. 4, and illustrates the manner whereby in some drying processes the moisture signals 8,, and 8 can be derived directly from moisture gauges and fed into the data processing system of FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS ported by conveyors through a drying unit. Another example lies in measuring the amount of liquid, solvents and water, in a cellophane film product.

In a specific embodiment, a dryer unit 10 is illustrated and may comprise a stema-operated dryer section or press rolls, or any other unit for removing moisture from a moving sheet 12. The sheet 12 enters the dryer [0 at location A and leaves at location B. These may be referred to as the wet end and the dry end, respectively. The dryer l removes moisture from the entering sheet and provides a relatively dry sheet at the far end. For this purpose, the dryer receives a supply of heat from unit 14 for the purpose of evaporating moisture such as water from the sheet 12 as it passes from left to right. Frequently, the dryer 10 is provided with a plurality of individual sections 10a extending in the cross-machine direction; each section serves to remove water from one of several zones numbered from 1 to 8 and extending across the width of the dryer.

As a result of the addition of heat to the sheet, the moisture is reduced from a relatively high value at the wet end, A, to a low value at the dry end, B, as illustrated by the drying curves in FIG. 2. For example, the percent moisture at the wet end in a typical paper making process may approach 65 to 75 percent, while the percent moisture of the paper sheet leaving the dryer may be in the vicinity of 5 to 7 percent moisture. The curves in FIG. 2 may be representative of dryer operation along any selected zone 1 through 8.

The ordinates of the graph in FIG. 2 may be in units denoting the amount of moisture or water in the sheet 12. For example, such terms as moisture content, percent moisture, absolute moisture content, are commonly used. It is convenient to refer to the actual amount of water in the sheet at the wet end as the load on the dryer. This may be expressed in units of pounds of water per square foot and is the factor which is examined and compared with the final sheet moisture to establish the dryers performance.

Two different moisture loads may produce the same percent moisture, if the solid or fiber content changes. One may measure the percent moisture of the incoming sheet and use this paramater for comparison, if the weight of the solid or fiber in the sheet remains substantially constant. In other words, under this condition, the percent moisture is proportional to the load on the dryer. In this disclosure, the terms moisture content or water content are used synonomously hereinafter in referring to the load on the dryer and the percent moisture of the final sheet. Alternatively, the actual amount of water in the final sheet may be measured and compared to obtain the desired performance index.

Since it is the actual amount of incoming water that the dryer must handle, it is useful to determine its response to a predetermined moisture load. An acceptable performance results when the dryer encounters a relatively high moisture load at the wet end and delivers a sheet having a relatively low percent moisture. The dryer performs poorly when it produces a relatively wet sheet in response to a relatively light moisture load.

Very often the operation and general performance of the dryer l0 deteriorates due to mechanical malfunction or clogging that arise in the flow path. As a result, the performance of the dryer may vary considerably from time to time. Curves P and G graphically illustrate how the performance of the dryer may vary. Curve G starts at a relatively high moisture content or load and bottoms out at a relatively low moisture content and may be therefore indicative of a good or acceptable drying performance. On the other hand, curve P starts at a relatively low wet-end moisture content or load and levels out at a final dry-end moisture content greater than curve G. Curve P is representative of a poor or unacceptable drying performance. The remaining curves are representative of two different cases wherein the performance is indeterminate.

An indication of dryer performance is obtained by comparing how the moisture content or load at the wet end deviates from a pre-dryer target T with how the moisture at the dry end deviates from a post-dryer target T Signals S and S indicative of the sheet moisture at the wet end and at the dry end respectively, are computed by a computer 16 (see FIG. 1) from measurements made by gauges 18, 20 and 22 positioned before and after the dryer 10.

Gauge 18 measures the weight per unit area of the wet sheet and gauge 20 measures the total weight per unit area of the dried sheet. The gauge 22 provides a signal proportional to the moisture content m of the partially dried sheet. Since the moisture content of the entering sheet is very high and not readily monitored reliably by present state-of-the-art moisture gagues, its value is determined by subtracting the weight of the solid or fiber portion from the total wet basis weight or weight per unit area of the sheet. For example, the fiber basis weight will not usually vary substantially in the machine direction; its value will be where W is the total basis weight of the partially dried sheet, and m is the percent moisture in the dried sheet. The pre-dryer moisture content in pounds of water per pounds of paper, for example, is then qA (W-I B/ )I H) where W, is the total basis weight of the wet sheet entering the dryer 10. The moisture load W is u' /l l: B/ 11 (2) Other approximations or simplifications may be made in the computation of W depending on the relative levels of moisture encountered for different processes and taking into account the degree of accuracy which is desired. Signals S and 5,, are proportional to the quantities W and "1 Alternatively, transducers other than basis weight gauges may be employed to derive the desired sheet moisture signals 5,, and S FIG. 8, for example, illustrates a simplified system wherein the moisture content is directly measurable at both the wet and dry ends by moisture gauges 22a and 22 respectively, which directly provide the appropriate signals S, S

The basis weight gauges 18 and 20 may preferably each comprise a nucleonic gauge of the type described in U. S. Pat. No. 2,790,945, issued April 30, 1957 to H. R. Chope and assigned to the same assignee as the present invention. These gauges respond to the radiation passed through the sheet 12 by a source of radiation and provide an output signal proportional to the basis weight of the sheet 12.

The moisture gauge 22 may take many forms, such as for example, a detector that responds to thermalized neutrons emitted from hydrogen in the sheet moisture, a dielectric, or capacitance gague. A- preferred capacitance moisture gauging system is described in U. S. Pat. No. 3,155,901 issued to A. F. G. Hanken and U. 5. Pat. No. 3,155,902 issued to G. W. Walls, both patents issued on Nov. 3, 1964 and are assigned to the same assignee as the present invention. An electrode assembly is placed against the sheet and energized by one or more signal frequencies to determine the response and provide an output signal proportional to the moisture content of the sheet 12 in the form of percent moisture by weight in the sheet.

The moisture signals 8 and 5,; are supplied to a moisture data processing unit 24 which compares these signals with a pre-dryer target and a post-dryer target signal respectively, set in on lines 26 and 28. The target signals may be generated by a voltage divider circuit comprising a battery and a manually adjustable potentiometer and correspond to the values shown in FIG. 2. The construction of the data processor 24 will be described in detail hereinafter with reference to FIGS. 4 and 5. The data processor 24 may employ digital or analog signal processing techniques to provide the desired output indication.

Briefly, the target signals are subtracted from the moisture signals S and S to obtain a pair of deviation signals. The relative sense of the deviation signals is measured by a phase sensitive circuit to provide an output signal representative of dryer performance.

A drying performance indicator 30 receives the output signal from the moisture data processor 24 to provide an output, such as a typewritten record or other visual display 30a.

The system of FIG. 1 operates generally in the following manner: The data processor 24 computes the deviation in moisture content from the preselected target for each location A and B. The sense of each moisture deviation relative to its target is determined. The sense of the pre-dryer deviation is then compared with the sense of the post-dryer deviation and the following rules of logic are observed. If the sense of the two deviations are the same, there is a positive correlation and no absolute information can be determined. This results in an indeterminate performance situation. In other words, no reliable indication of the dryers operating performance is possible when the sense of the deviations at the dry end is the same as it is at the wet end.

If, however, there is a negative correlation between the sense of the post-dryer and sense of the pre-dryer deviations, i.e., they are in opposite directions, the data processor 24 generates either a poor" drying signal or a good drying signal. A good" drying signal is generated whenever the pre-dryer deviation in moisture or load is greater than target T and the postdryer moisture deviation is less than target T A poor drying signal is generated whenever the prc-dryer deviation in moisture or load is less than target and the post-dryer moisture deviation is greater than target T One or both moisture deviations may be equal to their respective target values, in which case, a poor" indication may be made. As a rule, it is generally desirable to display a "good" performance only when it isclearly indicated.

Since it is desirable to know how the performance varies from zone-to-zone, a scanning controller 32 may be used to traverse the gauges back and forth across the width of the sheet 12 by means of motors 34 and 36. Alternatively, a plurality of gauges may be employed. The gauges may be maintained in synchronism to measure essentially the same longitudinal zone extending down the sheet in the machine direction. Alternatively, the signal from gauge 18 can be delayed and the gauges arranged to have computer 16 receive signals from the same measured area that are compared simultaneously, to eliminate the effect of transport distance between gauges l8 and 20. It is assumed. that the fiber weight per unit area of the sheet will remain substantially constant in the machine direction. A signal may be coupled from the scanning controller 32 to the data processor 24 over line 38 which is indicative of the crosssheet position of the gauges. The gauge position signal may be used to synchronize the readout of indicator 30 with moisture measurements being made. This enables an indication of the dryers performance along each of the respective zones 1-8. The indicator 30 may be provided with a separate display such as a red lamp 40 and a green lamp 42 to indicate either a poor or a good drying performance respectively for each zone.

Referring now to FIGS. 3 and 4, since there may be a drift in sheet moisture over a period of time, the preselected targets T and T may not be representative. If fixed targets are employed, deviation signals of the same sense may result because the sheet moisture varies only in one direction from the target. Accordingly, it is preferred to average the sheet moisture content before and after the dryer and to use this value for purposes of comparison. Now, variations will occur on each side of the average resulting in deviation signals that can vary in sign across the width of the sheet 12. Typical cross-sheet moisture variations or profile as viewed looking upstream are illustrated in FIG. 3. The respective pre-dlryer and post-dryer averages are drawn in dotted lines 48 and 50. A graphical examination of the moisture profile drawn in FIG. 3 shows that three different types of dryer performance exist across the width of the sheet. The dryer per formance is indeterminate in zones 1 and 2, poor" in zones 3, 4 and 5, and good in zones 6, 7 and 8. This determination is made by observing the sense of the moisture deviations relative to the average values in accordance with the rules of logic stated above in reference to FIGS. 1 and 2.

By observing the performance indication, the operator is continuously advised of the dryers operation. He is able to locate malfunctions that detract from the dryers performance. The operator is provided with a maintenance tool that enables him to make adjustments to the dryer to optimize its drying performance. The profile indication particularly facilitates the adjustment of individual dryer sections [0a.

In FIG. 4, the moisture signals S, and S from the moisture content computer 16 are coupled to a pair of averaging devices 52, 54 whose operation is controlled by the scanning controller 32. This enables the determination of the average moisture content each time the gauges scan across the sheet. The averaging devices 52, 54 are energized as gauges start at one side of the sheet and are deenergized when the gauges reach the opposite side of the sheet. The variations in sheet moisture content are measured during each scan. Reference may had to U. S. Pat. No. 3,015,129, issued on Jan. 2, 1962 to W. C. Hays et al., and assigned to the same assignee as the present invention, for a more detailed description of the profile averaging techniquie described briefly herein.

Each averaging device 52, 54 may be provided with a signal holding or storage unit 520, 540 which may store the computed scan average or other representative value for a period of one or more subsequent scans. During the measuring scans, the variations in crosssheet moisture or water content are compared by means of comparators 56 and 58 with the stored value of cross-machine average sheet moisture. Comparator units 56 and 58 may be simply subtraction devices that provide an output signal proportional to the difference between two input signals. Comparator 56 provides an output signal W having a polarity in accordance with the sense or deviation of the measured wet-end moisture content from the average signal stored by an averaging device 52a. Comparator 58 provides an output signal D having a polarity indicative of the sense or deviation of the measured dry-end moisture content from the average value computed by averaging device 54.

The data processor 24 detects the polarity or sense and the magnitude of the deviation signals W and D. A typical data processing circuit 24 may include a pair of multipliers 60 and 62 and four diodes 63-66 coupling the multipliers to the wet and dry deviation signals W and D. The output of multiplier 62 is coupled directly to a summing amplifier 68. The phase inverting amplifier 70 couples the output of multiplier 60 to the input of the summing amplifier 68. An indicator 72 having a dial calibrated plus, zero and minus serves to indicate whether the performance of the dryer is acceptable, indeterminate or unacceptable respectively. Other sense comparison circuits such as phase detectors may be employed to respond to the respective polarities of the signals W and D and provide one of the three alternative indications of dryer performance.

A quantitative indication of how well or how badly the dryer is performing may be obtained by examining the amplitude of the signal provided by amplifier 68. Accordingly, an amplitude detector 71, such as linearizing square root computer, may be employed t provide an output signal which is proportional to th? relative amplitude of the deviation signals W and D. A separate meter 73 may be used to visually indicate the amplitude of this output signal. Alternatively, one meter may be constructed to accomplish the function of both of the meters 72 and 73. Other amplitude responsive circuits and display units will be apparent to those skilled in the art.

The operation of the scan averaging embodiment of the present invention proceeds as follows: the scanning controller 32 energizes the traversing motors 34 and 36 (see FIG. 1) to move the basis weight and moisture gauges across the sheet from one side to the other. The operation is first described in terms of a single point performance gauge thereby facilitating the explanation of the later extension to a profile readout. The automatic scanning routine of the scanning controller may be interdicted by a single point selector unit 74. This may be done by an operator selecting one of the single point positions 1 through 8 by means of a control knob 76 coupled to the selector unit 74 connected to the scanning controller. For pu poses of illustration, if it is assumed that zone 4 is to be monitored, the gauges will automatically position themselves along this machine direction zone. The variations in moisture content at both the wet and the dry end are averaged by the averaging devices 52 and 54. When the far side of the sheet is reached, the scanning controller 32 stops the movement of the gauges as well as the averaging of the moisture signals. The gauges are automatically returned to the opposite side of the sheet to start a new scan. The moisture signals developed during the next scan are compared by comparator units 56 and 58 with the average values computed for the preceding scan and stored by units 520 and 54a.

For purposes of illustration, assume a first condition where the moisture content of the sheet entering the dryer is relatively high, at least greater than the average cross-sheet value of the moisture content. Furthermore, let us assume that the moisture content of the sheet leaving the dyrer is less than the post-dryer average moisture content. Under these condictions, when the gauges reach zone 4, the moisture signal on line 78 connected to the positive input in comparator 56 will be greater than the average value retrieved from averaging device 52. This results in a deviation signal W having a positive polarity. Since the post-dryer moisture content is relatively small, the signal on line 80 connected to comparator 58 will be less than the average signal retrieved from averaging device 54. This results in a post-dryer deviation signal D which has a negative polarity.

By virtue of the polarizing effect of the diodes 6366,

it is apparent that the multiplier 60 receives a voltage on both input terminals, one of one polarity and another of the opposite polarity. The product developed by multiplier 60, of course, will be negative polarity which is reversed by the phase-reversing amplifier 70. Since there is no output from multiplier 62, due to back-biasing of diodes 65 and 66, summing amplifier 68 develops a positive-going output signal. The dryer 10, at least along zone 4, is operating according to curve G shown in FIG. 2, which is an acceptable performance and which is reflected by the positive indication upon indicator 72.

If, on the other hand, the moisture content at the wet end is less than the average value stored in averaging device 52a and the moisture content at the dry end and ,is greater than the average value stored in the averaging device 54a, the polarity of the deviation signals W and D developed by comparators 56 and 58, respectively, will be reversed from what they were in the previous example. In this case, the wet-end deviation signal W will be negative and the dry-end deviation signal D will be positive. Signals of this polarity are coupled only by diodes 65 and 66. Therefore, only multiplier 62 receives input signals to form a product on line 82. The polarity of this product, of course, is negative. Since only one input signal is provided to the summing amplifier 68, its output will be negative in polarity and the indicator 72 will register an unacceptable performance for the dryer 10.

In each of these two cases, meter 73 reflects the degree to which the performance is acceptable (or unacceptable). The quality of performance is directly proportional to the magnitude of the signals W and D. For example, if their magnitudes are relatively large, the performance indicated is very good or very poor depending on their respective polarities.

If both the pre-dryer moisture content and the postdryer moisture content are greater than the averages computed by averaging devices 52 and 54, respectively, it can be seen that the deviation signals W and D will have a positive polarity. Conversely, if the pre-dryer and post-dryer moisture contents are less than their respective average values, then the deviation signals W and D will both have a negative polarity. In either case, each multiplier receives only one input voltage. Since there is zero voltage on the other input to the multipliers, the computed product is zero/This indication is made upon indicator 72, reflecting an indeterminate dryer performance. In this case, no indication is made upon meter 73.

Instead of evaluating only one zone along the sheet, it. may be desirable to use a plurality of indicators 72 and 73, one set for each zone. The control knob 76 may be moved to an AUTO mode whereby the scanning controller 32 continually moves the gauges back and forth across the sheet and the meters 72 are automatically switched to data processor 24 to indicate the performance of the dryer along each of the zones 1-8. Means may be provided to maintain the readings of each indicator while the gauge scans past a particular zone and until it returns on the following scan. This signal-holding means will be apparent to those skilled in the art. It may be desirable to compute the average moisture content across the sheet only once every several scans, depending on how frequently the moisture content varies in the machine direction. How

often the average must be updated will be a function of the particular process being monitored.

Alternative forms of data processing and display are illustrated in FIGS. 5, 6 and 7.

With reference now to FIG. 5, it may be desirable to use a green lamp 84, a yellow lamp 86 and a red lamp 88 to indicate an acceptable, an indeterminate, or an unacceptable dryer performance respectively. These indicators are coupled by diodes 9093 to the deviation signals W and D by way of AND circuits 94 and 96 and a NOR circuit 98. The output of AND circuit 94 is connected to the green indicator lamp 84 and to one input of NOR circuit 98. The output of AND circuit 96 is connected to the red indicator lamp 88 and to the other input of NOR circuit 98. The output of the NOR circuit 98 is connected to the yellow'indicator lamp 86. The AND and NOR circuits are well known to those skilled in the art. It may be necessary to employ inverter circuits I and I if AND circuits 94 and 96 accept only input signals having the same polarity. The operation of this circuit may be examined along the lines used in explaining the circuit in FIG. 4. For example, if the dryer performance is acceptable, the wet-end deviation signal W will be positive and the dry-end deviation signal D will be negative. This results in signals applied simultaneously to the AND circuit 94. AND circuit 94 provides an output signal which energizes the green indicator lamp 84.

[f the dryer performance deteriorates, the wet-end deviation signal W will be negative and the dry-end deviation signal D will be positive, causing AND circuit 96 to energize the red indicator lamp 88.

If, on the other hand, deviation signals W and D have the same polarity, each AND circuit will receive only one input signal and therefore fail to energize either the green or the red indicator lamps. Since NOR circuit 98 receives no signal on either of its inputs, it will provide an output signal energizing the yellow indicator lamp 86, reflecting the indeterminate dryer performance.

Of course, if more than one zone across the sheet must be monitored, a bank of indicator lights'may be automatically switched in to the logic circuitry in accordance with the position of the gauges across the sheet.

In some cases, it may be more desirable to use a chart recorder such as shown in FIG. 6 to indicate the performance profile for the dryer 10. In this alternative embodiment, a marking indicator 100 is coupled to the scanning controller 32 so that its lateral position from left to right across the chart 102 is indicative of the position of the scanning gauges across the width of the sheet 12. This may simply be a repeat slidewire coupled to the gauge positioning servo circuits used in the scanning controller unit 32. The recorder scale 103 may be labeled to designate the cross-sheet zones l-8.

A chart drive unit 104 receives signals from the AND and NOR circuits shown in FIG. 5, and translates these into a rotation displacement of a chart roll 106. For example, the signal from the AND circuit 94 may be connected to an input 104a to cause the chart drive to move the drive roll 106 down. The output signal from the AND circuit 96 may be coupled to input terminal 104!) and used to step the chart roll 106 up in the opposite direction through a fixed angular displacement. The output signal from the NOR circuit 98 is connected to input line 104e, which keeps the chart roll 106 in the center of its angular displacement range. The trace made by marking indicator 100 upon chart 102 may be interpreted as follows: a trace falling below a base line 108 is indicative of a deteriorated performance, and a trace falling above the base line is indicative of an acceptable performance. It may be noted that an acceptable performance signal provided by AND circut 94 will step the chart 102 down causing the drawn trace to fall above the base line 108. The converse is true it an unacceptable performance signal is developed by AND circuit 96 on line 104b. An output signal from the NOR signal will of course maintain the sheet along the base line 108 which is drawn during the time the regions return from one side of the-sheet to the other. At the end of every scan of the sheet, a scan index unit 110 may be used to step the chart drive 104 through a relatively large angular displacement to move the chart down to separate the individual scan traces. Reference may be had to U. S. Pat. No.

3,108,844 issued on Oct. 29, 1963 to F. M. Alexander et al., assigned to the same assignee as the present invention, for a more detailed description of x-y profile readout circuitry.

What is provided is an .r-y profile indication of the performance of the dryer across the width of the sheet. Alternatively, instead of causing the chart roll 106 to step up and down in accordance with the signals provided by the AND circuits 94 and 96, it may be more desirable to employ a pair of pens containing colored inks to distinguish between the dryer performance conditions. Referring to FIG. 7, a chart recorder 114 is provided with a green pen 116 and a red pen 118 individually positioned against the surface of the chart. A green pen 116 may be energized by a green pen selector 120 connected to the output of AND circuit 94. A red pen selector 122 may be coupled to the output of AND circuit 96 to position the red pen 118 against the chart. The pens may be mounted on one housing movable simultaneously with the gauges as described above in reference to FIG. 6. A green trace upon the chart is indicative of an unacceptable performance for other zones through which it is drawn. in the absence of a signal from either of the AND circuits 94 and 96, neither pen is positioned on the chart and a gap or space 124 will be indicative of an indeterminate dryer performance for those zones where the gap exists. Chart drive and indexing units 126 and 128 may also be employed.

Other circuits for electronically processing the moisture data signals and displaying the performance indices for the operator will be apparent to those skilled in the art.

For example, in certain applications it may be necessary to use a delay network 130 (see FIG. 4) to delay the pre-dryer signal so that signals representative of the same area of the sheet are compared.

it may also be desirable to use signal amplitude limiters 132 and 134 (see FIG. to prevent saturating the logic circuits. Other techniques for processing the derived deviation signals W and D prior to the AND circuits will be apparent to those skilled in the art.

The profile deviation signals W and D may be recorded directly on a chart for the operator to visually compare and interpret. Regardless of the type employed, the dryer performance display facilitates the manual or automatic adjustment of a valve 14a (see FIG. 1) coupling the heat supply 14, for example, to the dryer to maintain an economically desirable single point or profile performance condition. Individual valves may be used to control the performance profile of the dryer 10.

Although certain specific embodiments of the invention have been shown and described herein, many modifications may be made thereto without departing from the true spirit and scope of the invention as set forth in the appended claims.

What is claimed is:

1. The method of evaluating the performance of a dryer in removing moisture from a moisture-laden material to provide a partially dried material, said method comprising the steps of:

measuring the pre-dryer moisture in said moistureladen material to provide a first signal indicative of any deviation of said moisture from a first reference value therefor, the sense of said first signal depending on the relative magnitudes of the measured moisture and said first reference value; measuring the post-dryer moisture of said partially dried material to provide a second signal indicative of any deviation of said moisture from a second reference value therefor, the sense of said second signal depending on the relative magnitudes of the measured moisture and said second reference value; and

comparing said first signal and said second signal to provide an output signal indicative of the relative senses or said first and second signals and hence indicative of the drying performance of said dryer.

2. The method of evaluating the performance of a dryer in removing moisture from a moisture-laden material to provide a partially dried material, said method comprising the steps of:

measuring any pre-dryer deviation of the moisture of said moisture-laden material from a first reference value therefor, measuring any post-dryer deviation of the moisture of said partially dried material from a second reference value therfor, and

comparing said measured deviations to provide an indication of acceptable performance when said pre-dryer deviation is an increase, and said postdryer deviation is a decrease, from the respective reference value.

3. The method of evaluating the performance of a dryer in removing moisture from a moisture-laden material to provide a partially dried material, said method comprising the steps of:

measuring any deviations of the pre-dryer moisture content of said moisture-laden material from a reference value therefor, measuring any deviation of the post-dryer moisture content of said partially dried material from a reference therefor, and

comparing said measured deviations to provide one indication when said pre-dryer moisture content is greater than said reference value therefor and said post-dryer moisture content is less than said reference value therefor, and a second indication when said pre-dryer moisture content is less than said reference value therefor and said post-dryer moisture content is greater than said reference value therefor.

4. The method of evaluating the performance of a dryer in removing moisture from a moisture-laden material to provide a partially-dried material, said method comprising the steps of:

measuring the pre-dryer moisture content of said moisture-laden material,

measuring the post-dryer moisture content of said partially-dried material,

computing the average moisture content of said moisture-laden material,

computing the average moisture content of said partially-dried material,

detecting any deviation of either of said moisture measurements from their respective computed average moisture contents, and

comparing the detected deviations of said pre-dryer and said post-dryer deviations to provide an indication of the drying performance of said dryer.

5. The method of evaluating the performance of a dryer in removing moisture from a moisture-laden material to provide a partially-dried material, said method comprising the steps of:

measuring the moisture content of said moistureladen material at a plurality of locations across the width of said material,

measuring the average moisture content of said moisture-laden material,

measuring the moisture content of said partiallydried material at substantially said same crossmaterial locations,

measuring the average moisture content of said partially-dried material,

computing a first deviation signal proportional to the difference between said measured moisture content of said moisture-laden material and the measured average value thereof, and a second deviation signal proportional to the difference between said measured moisture content of said partiallydried material and the measured average value thereof,

detecting the sense of said first and second deviation signals, and

comparing said detected signal senses to provide an indication of the performance of said dryer.

6. The method set forth in claim 5 which further includes detecting the magnitude of said first and second deviation signals, and

comparing said detected signal amplitudes to provide an output signal having a magnitude indicative of the quality of said dryer performance.

7. The method of evaluating the performance of a dryer in removing moisture from a moisture-laden sheet to provide a partially-dried sheet, said method comprising the steps of:

scanning substantially across the width of said moisture-laden sheet to measure the pre-dryer moisture profile of said sheet,

computing from said scanning measurement the average moisture content of said moisture-laden sheet,

combining said computed average moisture content and the moisture content measured at a plurality of selected locations across the width of said moisture-laden sheet to provide a first moisture profile deviation signal,

scanning substantially across the width of said partially-dried sheet to measure the post-dryer moisture profile of said sheet,

computing from said post-dryer moisture profile measurement the average value of said moisture content of said partially-dried sheet,

combining said computed average moisture content of said partially-dried sheet and the moisture content measured at a second plurality of locations across the width of said partially'dried sheet and corresponding to said selected pre-dryer locations across the width of said sheet for providing a second profile deviation signal, and combining said first and said second profile deviation signals for each of said corresponding locations to provide an indication of the drying performance of said dryer in those cross-sheet sections that are associated with said selected locations.

8. The method of evaluating the performance of a dryer in removing moisture from a moisture-laden sheet to provide a partially-dried sheet, said method comprising the steps of:

scanning substantially across the width of said moisture-laden sheet to measure the pre-drycr moisture profile of said sheet,

computing from said scanning measurement the average moisture content of said moisture-laden sheet, combining said computed average moisture content and the moisture content measured at one or more selected locations across the width of said moisture-laden sheet to provide a first profile deviation signal,

scanning substantially across the width of said partially-dried sheet to measure the post-dryer moisture profile of said sheet,

computing from said post-dryer moisture profile measurement the average value of said moisture content of said partially-dried sheet,

combining said computed average moisture content of said partially-dried sheet and the moisture content measured atone or more locations across the width of said partially-dried sheet corresponding to said selected predryer locations for providing a second profile deviation signal,

generating a first output signal when said pre-dryer moisture profile deviation signal has one sense and said post-dryer moisture profile deviation signal has the opposite sense and a second output signal when said pre-dryer moisture profile deviation signal has said opposite sense and said post-dryer moisture profile deviation signal has said other sense, and

utilizing said output signals to indicate the dryistg performance of said dryer. 1

9. In combination with a dryer unit for removing moisture from a moisture-laden material, a first moisture gauge means for measuring the pre-dryer moisture content of said material, a second moisture gauge means for measuring the post-dryer moisture content of said material, said gauges providing a first and a second signal respectively, means for providing a first and a second reference value for said pre-dryer and said post-dryer moisture content respectively, the improvement comprising:

means for computing the difference between said first gauge signal and said first reference signal and between said second gauge signal and said second reference signal to provide a third and a fourth signal respectively, said the said computing means including means responsive to the sense of said third signal relative to said fourth signal for providing an indication of the relative drying performance of said dryer.

10. The combination as in claim 9 in which said computing means further includes means responsive to the magnitude of said third signal relative to said fourth signal for providing an output signal having a mag- .nitude indicative of the quality of said dryer performance.

l l. The combination as in claim 9 in which said sense responsive means comprises:

logic means for providing a first performance indication when said sense between said first gauge means and said first target and the sense between said second gauge means and said second target are the same, a second performance indication when said first sense between said first gauge means and siad first target and the sense between said second gauge means and said second target are different in one sense, and a third performance indication when said sense between said first gauge means and said first target and the sense between said second gauge means and said second target are different in he other sense. 12. The combination as in claim 9 in which said sense response means comprises:

a first and a second multiplier, each having a pair of input terminals and an output terminal, means for coupling said first difference signal of one polarity and said second difference signal of opposite polarity to different input terminals of said first multiplier,

means for coupling said first difference signal of the other polarity and said second difference signal of opposite polarity to different input terminals of said second multiplier, and

means coupled to said multiplier output terminals for 5 providing at least one dryer performance signal in accordance with a predetermined phase relationship between said first difference signal and said second difference signal.

13. In combination with a dryer unit for removing moisture from moisture-laden material, a first moisture gauge means for measuring the pre-dryer moisture content of said material, a second moisture gauge' means for measuring the post-dryer moisture content of said material, said gauges providing a first and a second signal respectively, means for providing a first and a second reference signal representative of a pre-dryer and a post-dryer moisture content respectively, the improvement comprising:

means for computing the difference between said first gauge signal and said first reference signal and between said second gauge signal and said second reference signal to provide a third and a fourth signal respectively, and

means responsive to the sense of said third signal relative to the sense of said fourth signal for providing an indication of the relative drying performance of said dryer.

14. In combination with a first gauge means positioned before a dryer unit receiving a moisture-laden sheet to remove moisture therefrom and a second gauge means positioned after said dryer unit adjacent to the partially dried sheet provided by said dryer unit, said first and second gauge means providing a first and a second moisture gauge signal respectively, the improvement comprising:

means for. computing the average moisture content of said moisture-laden sheet and said partially dried sheet and providing a first and a second 40 average signal proportional thereto,

means for computing the difference between saij first gauge signal and said first average signal an between said second gauge signal and said second average signal to provide a first and a second difference signal, respectively, and means responsive to the polarity of said first difference signal relative to said second difference signal to provide an output indication representative of the drying performance of said dryer.

15. The combination as in claim 14 in which said polarity responsive means comprises:

logic circuit means for providing a first performance indication when said difference signals are of the same sign, a second performance indication when said first difference signal is one sign different from that of said second difference signal, and a third performance indication when said first difference signal is the other sign different from that of said second difference signal.

16. The combination as in claim 14 in which said output indication comprises an array of colored lights, each color designating a different dryer performance condition.

17. In combination with a first moisture gauge means positioned before a dryer unit receiving a moistureladen sheet to remove moisture therefrom, a second moisture gauge means positioned after said dryer unit adjacent to the partially dried sheet provided by said dryer unit, means for moving each of said gauge means in a scanning movement back and forth across said sheet to provide a first and a second gauge signal proportional to the moisture content of said sheet at the traversing point of measurement, the improvement comprising:

scan average computing means responsive to the movement of said gauge means for providing a first scan average signal proportional to the average pre-dryer moisture content of said sheet and a second scan average signal proportional to the average post-dryer moisture content of said sheet,

means for computing the difference between said first gauge signal and said first scan average signal and between said second gauge signal and said second scan average signal to provide a first and second difference signal respectively, and

means for comparing the sense of said first difference signal with the sense of said second difference signal to provide an indication of the drying performance of said dryer. 18. In combination with a first moisture gauge means positioned before a dryer unit receiving a moistureladen sheet to remove moisture therefrom and a second moisture gauge means positioned after said dryer unit adjacent to the partially dried sheet provided by said dryer unit, means for moving each of said gauge means in a scanning movement back and forth across said sheet to provide a first and a second gauge signal proportional to the moisture content of said sheet at the traversing point of measurement, the improvement comprising:

scan average computing means responsive to the movement of said gauge means for providing a first scan average signal proportional to the average pre-dryer moisture content of said sheet and a second scan average signal proportional to the average post-dryer moisture content of said sheet,

means for computing the difference between said first gauge signal and said first scan average signal and between said second gauge signal and said second scan average signal to provide a first and a second moisture profile difference signal indicative of moisture variations across the width of said sheet, and

means for comparing said moisture profile difference signals to provide an indication of the drying performance profile of said dryer.

19. The combination as in claim 18 in which said moisture profile difference signal comparing means comprises:

means for providing a third signal representative of the position of said gauge means across the width of said sheet,

means for comparing the phase of said first profile difference signal with the phase of said second profile difference signal to provide an output signal, and

a performance profile indicator comprising a bank of indicators, one designated for a different zone extending down the length of said sheet and means responsive to said third signal for energizing one of said indicators in accordance with said output signal to indicate the drying performance profile of said dryer.

20. The combination as in claim 18 in which said moisture profile difference signal comparing means comprises:

logic circuit means for providing an output signal indicative of the phase relationship between said first and second moisture profile difference signals.

21. The combination as in claim 18 in which said moisture profile difference signal first means comprises:

diode logic circuit means for providing an output signal in accordance with the phase relationship between said first and said second moisture profile difference signals, and

chart recorder means for providing an x-y plot of said output signal versus the cross-dimension of said sheet.

22. Apparatus for evaluating the performance of a dryer in removing moisture from a moisture-laden material to provide a partially dried material, comprisfirst means for measuring the pre-dryer moisture in said moisture-laden material and for providing a first signal indicative of any deviation of said moisture from a first reference value therefor, said first means including means for determining the sense of said first signal depending on the relative magnitudes of the measured moisture and said first reference value;

second means for measuring the post-dryer moisture of said partially dried material and for providing a second signal indicative of any deviation of said moisture from a second reference value therefor, said second means including means for determining the sense of said second signal depending on the relative magnitudes of the measured moisture and said second reference value; and third means responsive to said first signal and said second signal for producing an output signal indicative of the relative senses of said first and second signals and hence indicative of the drying performance of said dryer. 

1. The method of evaluating the performance of a dryer in removing moisture from a moisture-laden material to provide a partially dried material, said method comprising the steps of: measuring the pre-dryer moisture in said moisture-laden material to provide a first signal indicative of any deviation of said moisture from a first reference value therefor, the sense of said first signal depending on the relative magnitudes of the measured moisture and said first reference value; measuring the post-dryer moisture of said partially dried material to provide a second signal indicative of any deviation of said moisture from a second reference value therefor, the sense of said second signal depending on the relative magnitudes of the measured moisture and said second reference value; and comparing said first signal and said second signal to provide an output signal indicative of the relative senses or said first and second signals and hence indicative of the drying performance of said dryer.
 2. The method of evaluating the performance of a dryer in removing moisture from a moisture-laden material to provide a partially dried material, said method comprising the steps of: measuring any pre-dryer deviation of the moisture of said moisture-laden material from a first reference value therefor, measuring any post-dryer deviation of the moisture of said partially dried material from a second reference value therfor, and comparing said measured deviations to provide an indication of acceptable performance when said pre-dryer deviation is an increase, and said post-dryer deviation is a decrease, from the respective reference value.
 3. The method of evaluating the performance of a dryer in removing moisture from a moisture-laden material to provide a partially dried material, said method comprising the steps of: measuring any deviations of the pre-dryer moisture content of said moisture-laden material from a reference value therefor, measuring any deviation of tHe post-dryer moisture content of said partially dried material from a reference therefor, and comparing said measured deviations to provide one indication when said pre-dryer moisture content is greater than said reference value therefor and said post-dryer moisture content is less than said reference value therefor, and a second indication when said pre-dryer moisture content is less than said reference value therefor and said post-dryer moisture content is greater than said reference value therefor.
 4. The method of evaluating the performance of a dryer in removing moisture from a moisture-laden material to provide a partially-dried material, said method comprising the steps of: measuring the pre-dryer moisture content of said moisture-laden material, measuring the post-dryer moisture content of said partially-dried material, computing the average moisture content of said moisture-laden material, computing the average moisture content of said partially-dried material, detecting any deviation of either of said moisture measurements from their respective computed average moisture contents, and comparing the detected deviations of said pre-dryer and said post-dryer deviations to provide an indication of the drying performance of said dryer.
 5. The method of evaluating the performance of a dryer in removing moisture from a moisture-laden material to provide a partially-dried material, said method comprising the steps of: measuring the moisture content of said moisture-laden material at a plurality of locations across the width of said material, measuring the average moisture content of said moisture-laden material, measuring the moisture content of said partially-dried material at substantially said same cross-material locations, measuring the average moisture content of said partially-dried material, computing a first deviation signal proportional to the difference between said measured moisture content of said moisture-laden material and the measured average value thereof, and a second deviation signal proportional to the difference between said measured moisture content of said partially-dried material and the measured average value thereof, detecting the sense of said first and second deviation signals, and comparing said detected signal senses to provide an indication of the performance of said dryer.
 6. The method set forth in claim 5 which further includes detecting the magnitude of said first and second deviation signals, and comparing said detected signal amplitudes to provide an output signal having a magnitude indicative of the quality of said dryer performance.
 7. The method of evaluating the performance of a dryer in removing moisture from a moisture-laden sheet to provide a partially-dried sheet, said method comprising the steps of: scanning substantially across the width of said moisture-laden sheet to measure the pre-dryer moisture profile of said sheet, computing from said scanning measurement the average moisture content of said moisture-laden sheet, combining said computed average moisture content and the moisture content measured at a plurality of selected locations across the width of said moisture-laden sheet to provide a first moisture profile deviation signal, scanning substantially across the width of said partially-dried sheet to measure the post-dryer moisture profile of said sheet, computing from said post-dryer moisture profile measurement the average value of said moisture content of said partially-dried sheet, combining said computed average moisture content of said partially-dried sheet and the moisture content measured at a second plurality of locations across the width of said partially-dried sheet and corresponding to said selected pre-dryer locations across the width of said sheet for providing a second profile deviation signal, and combining said first and said second profile deviation signals for each of Said corresponding locations to provide an indication of the drying performance of said dryer in those cross-sheet sections that are associated with said selected locations.
 8. The method of evaluating the performance of a dryer in removing moisture from a moisture-laden sheet to provide a partially-dried sheet, said method comprising the steps of: scanning substantially across the width of said moisture-laden sheet to measure the pre-dryer moisture profile of said sheet, computing from said scanning measurement the average moisture content of said moisture-laden sheet, combining said computed average moisture content and the moisture content measured at one or more selected locations across the width of said moisture-laden sheet to provide a first profile deviation signal, scanning substantially across the width of said partially-dried sheet to measure the post-dryer moisture profile of said sheet, computing from said post-dryer moisture profile measurement the average value of said moisture content of said partially-dried sheet, combining said computed average moisture content of said partially-dried sheet and the moisture content measured at one or more locations across the width of said partially-dried sheet corresponding to said selected pre-dryer locations for providing a second profile deviation signal, generating a first output signal when said pre-dryer moisture profile deviation signal has one sense and said post-dryer moisture profile deviation signal has the opposite sense and a second output signal when said pre-dryer moisture profile deviation signal has said opposite sense and said post-dryer moisture profile deviation signal has said other sense, and utilizing said output signals to indicate the drying performance of said dryer.
 9. In combination with a dryer unit for removing moisture from a moisture-laden material, a first moisture gauge means for measuring the pre-dryer moisture content of said material, a second moisture gauge means for measuring the post-dryer moisture content of said material, said gauges providing a first and a second signal respectively, means for providing a first and a second reference value for said pre-dryer and said post-dryer moisture content respectively, the improvement comprising: means for computing the difference between said first gauge signal and said first reference signal and between said second gauge signal and said second reference signal to provide a third and a fourth signal respectively, said the said computing means including means responsive to the sense of said third signal relative to said fourth signal for providing an indication of the relative drying performance of said dryer.
 10. The combination as in claim 9 in which said computing means further includes means responsive to the magnitude of said third signal relative to said fourth signal for providing an output signal having a magnitude indicative of the quality of said dryer performance.
 11. The combination as in claim 9 in which said sense responsive means comprises: logic means for providing a first performance indication when said sense between said first gauge means and said first target and the sense between said second gauge means and said second target are the same, a second performance indication when said first sense between said first gauge means and siad first target and the sense between said second gauge means and said second target are different in one sense, and a third performance indication when said sense between said first gauge means and said first target and the sense between said second gauge means and said second target are different in he other sense.
 12. The combination as in claim 9 in which said sense response means comprises: a first and a second multiplier, each having a pair of input terminals and an output terminal, means for coupling said first difference signal of one polarity and said second difference signal of opposite polarity to diFferent input terminals of said first multiplier, means for coupling said first difference signal of the other polarity and said second difference signal of opposite polarity to different input terminals of said second multiplier, and means coupled to said multiplier output terminals for providing at least one dryer performance signal in accordance with a predetermined phase relationship between said first difference signal and said second difference signal.
 13. In combination with a dryer unit for removing moisture from moisture-laden material, a first moisture gauge means for measuring the pre-dryer moisture content of said material, a second moisture gauge means for measuring the post-dryer moisture content of said material, said gauges providing a first and a second signal respectively, means for providing a first and a second reference signal representative of a pre-dryer and a post-dryer moisture content respectively, the improvement comprising: means for computing the difference between said first gauge signal and said first reference signal and between said second gauge signal and said second reference signal to provide a third and a fourth signal respectively, and means responsive to the sense of said third signal relative to the sense of said fourth signal for providing an indication of the relative drying performance of said dryer.
 14. In combination with a first gauge means positioned before a dryer unit receiving a moisture-laden sheet to remove moisture therefrom and a second gauge means positioned after said dryer unit adjacent to the partially dried sheet provided by said dryer unit, said first and second gauge means providing a first and a second moisture gauge signal respectively, the improvement comprising: means for computing the average moisture content of said moisture-laden sheet and said partially dried sheet and providing a first and a second average signal proportional thereto, means for computing the difference between said first gauge signal and said first average signal and between said second gauge signal and said second average signal to provide a first and a second difference signal, respectively, and means responsive to the polarity of said first difference signal relative to said second difference signal to provide an output indication representative of the drying performance of said dryer.
 15. The combination as in claim 14 in which said polarity responsive means comprises: logic circuit means for providing a first performance indication when said difference signals are of the same sign, a second performance indication when said first difference signal is one sign different from that of said second difference signal, and a third performance indication when said first difference signal is the other sign different from that of said second difference signal.
 16. The combination as in claim 14 in which said output indication comprises an array of colored lights, each color designating a different dryer performance condition.
 17. In combination with a first moisture gauge means positioned before a dryer unit receiving a moisture-laden sheet to remove moisture therefrom, a second moisture gauge means positioned after said dryer unit adjacent to the partially dried sheet provided by said dryer unit, means for moving each of said gauge means in a scanning movement back and forth across said sheet to provide a first and a second gauge signal proportional to the moisture content of said sheet at the traversing point of measurement, the improvement comprising: scan average computing means responsive to the movement of said gauge means for providing a first scan average signal proportional to the average pre-dryer moisture content of said sheet and a second scan average signal proportional to the average post-dryer moisture content of said sheet, means for computing the difference between said first gauge signal and said first scan average signal and between said second gauge signal and said sEcond scan average signal to provide a first and second difference signal respectively, and means for comparing the sense of said first difference signal with the sense of said second difference signal to provide an indication of the drying performance of said dryer.
 18. In combination with a first moisture gauge means positioned before a dryer unit receiving a moisture-laden sheet to remove moisture therefrom and a second moisture gauge means positioned after said dryer unit adjacent to the partially dried sheet provided by said dryer unit, means for moving each of said gauge means in a scanning movement back and forth across said sheet to provide a first and a second gauge signal proportional to the moisture content of said sheet at the traversing point of measurement, the improvement comprising: scan average computing means responsive to the movement of said gauge means for providing a first scan average signal proportional to the average pre-dryer moisture content of said sheet and a second scan average signal proportional to the average post-dryer moisture content of said sheet, means for computing the difference between said first gauge signal and said first scan average signal and between said second gauge signal and said second scan average signal to provide a first and a second moisture profile difference signal indicative of moisture variations across the width of said sheet, and means for comparing said moisture profile difference signals to provide an indication of the drying performance profile of said dryer.
 19. The combination as in claim 18 in which said moisture profile difference signal comparing means comprises: means for providing a third signal representative of the position of said gauge means across the width of said sheet, means for comparing the phase of said first profile difference signal with the phase of said second profile difference signal to provide an output signal, and a performance profile indicator comprising a bank of indicators, one designated for a different zone extending down the length of said sheet and means responsive to said third signal for energizing one of said indicators in accordance with said output signal to indicate the drying performance profile of said dryer.
 20. The combination as in claim 18 in which said moisture profile difference signal comparing means comprises: logic circuit means for providing an output signal indicative of the phase relationship between said first and second moisture profile difference signals.
 21. The combination as in claim 18 in which said moisture profile difference signal first means comprises: diode logic circuit means for providing an output signal in accordance with the phase relationship between said first and said second moisture profile difference signals, and chart recorder means for providing an x-y plot of said output signal versus the cross-dimension of said sheet.
 22. Apparatus for evaluating the performance of a dryer in removing moisture from a moisture-laden material to provide a partially dried material, comprising first means for measuring the pre-dryer moisture in said moisture-laden material and for providing a first signal indicative of any deviation of said moisture from a first reference value therefor, said first means including means for determining the sense of said first signal depending on the relative magnitudes of the measured moisture and said first reference value; second means for measuring the post-dryer moisture of said partially dried material and for providing a second signal indicative of any deviation of said moisture from a second reference value therefor, said second means including means for determining the sense of said second signal depending on the relative magnitudes of the measured moisture and said second reference value; and third means responsive to said first signal and said second signal for producing an output signal indicative of the relative sEnses of said first and second signals and hence indicative of the drying performance of said dryer. 